Condensation reaction between urea and formaldehyde has been known for many years. Urea formaldehyde based insulation foams have been commercially available in the United States since at least as early as the 1950's. They were, however, at that time not widely used. In recent times, with the emphasis on energy conservation, there has been an increasingly greater use of these materials for residential insulation, and in particular, as a foamed insulation to be used in wall spaces and the like, of existing building structures.
With the increasing use of urea formaldehyde foamed insulation to increase insulating capacity, there has been concern expressed about off-gassing of formaldehyde from the insulation, after installation. It appears that temperature and humidity affect the levels of formaldehyde off-gassing. Also, there appear to be two emission phenomena occurring. The first involves a short term release of formaldehyde and the second involves a lower level, long term release.
The release of formaldehyde from the resin into the interior of a dwelling structure is undesirable. It has been linked with certain physical and biological effects. This is particularly true for low level, long term release of out-gassed formaldehyde which may expose persons inside of the insulated dwelling to potential long term risks. In fact, because of such concerns, urea formaldehyde foamed insulation was at one time banned for use in the United States.
Thus, while urea formaldehyde is unquestionably effective from the insulating standpoint and as an adhesive resin, if it is to be used in the future, there is a real and continuing need for developing such which will eliminate or minimize the out-gassing of formaldehyde, both from the standpoint of short term release and most importantly, from the standpoint of the lower level, long term gradual release.
Urea formaldehyde resins are a mixture of products from the condensation reaction between urea and formaldehyde. The first stage of the condensation is commonly carried out at 70.degree. to 100.degree. C. with a urea formaldehyde ratio of approximately 1.5 and a pH of from about 4 to about 8. This results in a mixture of mono and di-methylol urea. The product of this first condensation takes on a variety of forms which are dependent upon reaction conditions such as pH, temperature and the mole ratio of urea to formaldehyde. They can be prepared as syrupy, aqueous solutions, water soluble fine grain powders, or water insoluble materials which precipitate from the reaction solution. The urea formaldehyde resin mixture previously described is often used for an adhesive in making particle board.
The particle board will out-gas formaldehyde as it is being slowly released from the adhesive. As a result, there has been some considerable recent concern about using urea formaldehyde as an adhesive resin for particle board, in spite of its known effectiveness. Such adhesives have been used for the bonding of wood in the formation of laminated beams, plywood, fiber boards, wood moldings and particle boards for more than 40-50 years. In the United States, about 75% of all UF resin is used as a wood adhesive, and 85% of this is consumed by the particle board industry.
Particle board, like UF foam, has problems with formaldehyde emission. In my prior issued patent, U.S. Pat. No. 4,366,264, issued Dec. 28, 1982, it was reported that calcium metasilicate could be used for formaldehyde suppression in UF foams. It has now been discovered that this same additive can be effectively adapted for particle board.
In use as a particle board adhesive, urea formaldehyde resin has a very large surfaced volume ratio and the resin has a variable thickness and contact with surfaces which on a microscopic scale vary in chemical, physical and mechanical behavior.
Treatments of this problem are numerous. Paper overlays, plastic laminates, veneer and metal faces, chemical coatings and paints have all been used, but cost more than the treatment with additives which can be used before or during the formation of the boards or molding. Such additives bind or absorb the formaldehyde. There have been certain prior attempts at additives. For example, a mixture of mineral oil and sodium silicate is reported to diminish the evolution of formaldehyde from particle board in the following disclosure.
Z. F. Dashkovskaya, T. G. Zakharov, T. I. Kravchenko and K. I., Stankevich, USSR, No. P 480,555, Aug. 15, 1975.
In addition, in a report by M. Higuchi, et al., Mokuzai Gakkai-shi, 26, 310 (1980), glass powder with a composition of (Na.sub.2 O) 0.26(CaO) 0.22(SiO.sub.2) 1.0 B.sub.2 O.sub.3 (0.08) was used as an acid scavenger in urea formaldehyde bonded plywood and found in amounts up to 20 percent to improve the water resistance of the plywood. The shear strength of plywood decreases with time in spite of the presence of the glass powder.
The primary objective of the present invention is to modify urea formaldehyde resins so that these resins can be still used as effective particle board adhesive, but without urea formaldehyde resin detriments of significant out-gassing. The method and manner of achieving this primary objective, as well as others, will become apparent from the detailed description of the invention, which follows.